1. Field of the Invention
The present invention relates to a data modulation method and a data modulation device and a communication device, and more particularly is suitably applied to the digital communication system using such as the orthogonal frequency division multiplexing (OFDM) system to acquire synchronism of frame and packet by using synchronizing code series (hereinafter referred to as reference symbol) when receiving signals.
2. Description of the Related Art
In recent years, with the development of OFDM digital communication system, various standardization committees have been developing the standardization of these systems. And it is expected that the OFDM system digital communications systems standardized by these different standardization committees will be used in the same frequency band mixed in the near future.
In practice, in the digital communication system of OFDM system, the Institute of Electrical Electronics Engineers (IEEE) 802.11a formed the specification (P802.11a/D7.0 July 1999) and has been promoting for the standardization. And at the same time, the Broadband Radio Access Network (BRAN) has formed the specification (<DTS/BRAN03003-1>V0.j(1999-09)) and has been promoting for the standardization.
Furthermore, the High-speed Wireless Access Council/Wireless Access Group has already formed the specification “Multimedia Mobile Access Communication (MMAC) Systems specification Ver.1.1, H11.11.11 44-4” and been promoting for standardization, and simultaneously, the 5 GHz Band Mobile Access Special Council/Asynchronous Transfer Mode (ATM) Group forms “Multimedia Mobile Access Communication (MMAC) systems specification) Ver.1,1, H11.11.11 44-4” and has been working for its standardization.
In addition to these, presently the system to make the IEEE1394 to wireless, “wireless home link” as the digital communication system of the OFDM system, has been under the process of standardization in Japan.
The synchronization method in the digital communication system of the OFDM system standardized by the IEEE802.11a, BRAN, High-Speed Wireless Access Council/Wireless Access Group, and the 5 GHz Band Mobile Access Special Council/ATM Group described above has been already decided.
On the other hand, although in the synchronization method in the digital communication system of OFDM system using the wireless home link, the synchronization method has not been determined, it is proposed to define the reference symbols different from other 4 digital communication systems.
At this point, the structure of the conventional synchronization method and the reference symbol will be described. In the digital communication system, generally the transmitting side and the receiving side are operating in synchronism. More specifically, it is necessary to synchronize the timing and frequency between the transmitter and the receiver. For example, the symbol timing of the fast Fourier transform (FFT) unit should be synchronized.
In general, in order to synchronize the symbol having the specific timing sequence, that is the reference symbol, is to be transmitted. This reference symbol is transmitted overlaying onto the data to be transmitted.
In practice, as shown in FIG. 1, the reference symbol is interpolated to the head of transmission frame formed of F number of symbols, i.e., burst frame, as the head symbol and is transmitted. In this connection, guard intervals are provided between the reference symbol and the head data symbol #1, and between each of data symbols in order that interference between symbols (this is expressed as intersymbol interference, and hereinafter referred to as ISI) would not occur under the multi-bus condition.
Then, the identification of reference symbol in the receiving side, that is the timing in synchronism, will be determined according to the correlation on the time axis between the receiving signal in which reference symbol is included and the receiving signal delayed.
The maximum value of that correlation value is used to take synchronism in order to fit to the position of the last sample of the reference symbol as correctly as possible. In this connection, the reference symbol is formed of a plurality of synchronization patterns (hereinafter referred to as SP) in order to detect the maximum value of correlation value, and the SP appears several times in one reference symbol period.
FIGS. 2A and 2B show the construction or format of the reference symbol having the length N in which the SP with the length Nsp on the time axis is repeated N/Nsp times.
For example, in the digital communication system of the OFDM system, the symbol having the desired structure can be formed effectively by inverse fast Fourier transforming (hereinafter referred to as IFFT) the coefficient of the discrete Fourier transformation (hereinafter referred to as DFT).
Accordingly, in order to form the reference symbol of the length Ts having (N/Nsp) numbers of sync patterns to be expressed in the following equation, it is enough to modulate DFT coefficient of every (N/Nsp) order (sub-carrier of every (N/Nsp) order in the frequency region).
                              T          s                *                  [                                    N              sp                        N                    ]                                    (        1        )            
For example, when N=64, Nsp=16, only subcarrier of ±4, ±8. . . may be modulated.
In this connection, N shows the total number of samples in one reference symbol, i.e., one cycle of the reference symbol, and Nguard shows the number of samples in the guard section inserted to prevent the interference between symbols (ISI).
Moreover, the period of correlation window will be expressed as follows:N+Nguard−Nsp  (2)
Then, the correlation value R(i) in the time area of input data stream will be expressed in the following equation.
                              R          ⁡                      (            i            )                          =                              ∑                          n              -              0                                      N              +                              N                guard                            -                              N                sp                            -              1                                ⁢                                                    y                ⁡                                  (                                      i                    -                    n                                    )                                            ·              y                        *                          (                              i                -                n                -                                  N                  sp                                            )                                                          (        3        )            As is expressed above, the correlation value is, after multiplying the receiving signal by its conjugate complex signal, these are multiplied by the number of samples included in the correlation window.
At this point, the circuit construction for calculating the correlation value R(i) according to EQUATION (3) will be shown in FIG. 3. As this FIG. 3 shows, the input data (i.e., received data) is supplied to the delayer 81 and a multiplier 83, and the delayer 81 delays this input data for Nsp, i.e., 1 sync pattern, and supplies this to the conjugate complex function device 82.
The conjugate complex function device 82 supplies the input data delayed at the delayer 81 to the multiplier 83 searching for conjugate complex data. The multiplier 83 multiplies the input data by the conjugate complex data supplied from the conjugate complex function device 82, and supplies the resulting multiplied data to the delayer 84 and the adder 85.
The delayer 84, delaying the multiplied data for the period of correlation window, supplies this to the subtracter 86. This subtracter 86 is supplied with output data of the adder 85, and the subtracter 86 subtracts the multiplied data delayed at the delayer 84 from this output data, and supplies the resulting subtracted data to the delayer 87 having the delay time of 1 unit. The delayer 87, delaying the subtracted data, supplies this to the adder 85. The adder 85 adds the multiplied data from the multiplier 83 and the subtracted data delayed at the delayer 87 and supplies the resulting output data to the subtracter 86.
Thus, the received data is correlated with itself delayed for the period of sync pattern. And the correlation value R(i) is accumulated during the period of correlation window. The maximum value of the absolute value (|R(i)|) of the correlation value R(i), that is the output of the subtracter 86 is detected by the maximum value detection circuit (not shown in FIG. 3) Accordingly, the timing position of the last sample of the reference symbol to be received will be determined. Then, this time information is used as the symbol timing signal of the receiver.
In this case, the value of correlation R(i) expressed by the accumulation value shown in EQUATION (3) will become the maximum at a certain position to be described later. And by detecting the maximum value, the correct symbol timing in the receiver will be determined. In this connection, the detection of the maximum value of correlation value R(i) is conducted only by the reference symbol.
Furthermore, the determination whether the current symbol is the reference symbol or not is conducted based on the predetermined threshold value. More specifically, the maximum value of the absolute value |R(i)| while the absolute value |R(i)| of the correlation-value R(i) exceeds its threshold value is taken as the detection of reference symbol and the synchronization timing.
In this connection, the detection method of frequency offset and the effects of phase shift are described in the Japan Patent Application No. 10-330208 (Japan Patent Laid-Open No. 11-215097 bulletin) described above. And based on such method, the sync code series has been optimized and being used in the conventional digital communication system.
At this point, the generation method of the conventional reference symbol will be described in the following paragraph. All reference symbols are generated using 64-point IFFT. There are three kinds of complex number sequences in the input code sequence of IFFT, and these are named as SA, SB and SC respectively. In this connection, contents of SA, SB and SC will be shown in the EQUATION (4), EQUATION (5) and EQUATION (6) as follows:
                                          SA                                          -                26                            ,              26                                =                                                    (                                  13                  /                  6                                )                                      *                                                                                {                                          0                      ,                      0                      ,                      0                      ,                      0                      ,                      S1                      ,                      0                      ,                      0                      ,                      0                      ,                      S2                      ,                      0                      ,                      0                      ,                      0                      ,                                                                                                                                        S3                    ,                    0                    ,                    0                    ,                    0                    ,                    S4                    ,                    0                    ,                    0                    ,                    0                    ,                    S5                    ,                    0                    ,                    0                    ,                    0                    ,                                                                                                                    S6                    ,                    0                    ,                    0                    ,                    0                    ,                    S7                    ,                    0                    ,                    0                    ,                    0                    ,                    S8                    ,                    0                    ,                    0                    ,                    0                    ,                                                                                                                    S9                    ,                    0                    ,                    0                    ,                    0                    ,                    S10                    ,                    0                    ,                    0                    ,                    0                    ,                    S11                    ,                                                                                                                                          0                      ,                      0                      ,                      0                      ,                      S12                      ,                      0                      ,                      0                      ,                      0                      ,                      0                                        }                                                                                      ⁢                                  ⁢                              S1            ⁢                                                  ⁢            …            ⁢                                                  ⁢            12                    =                                                                                          (                                                                  -                        1                                            +                      j                                        )                                    ,                                      (                                          1                      +                      j                                        )                                    ,                                      (                                                                  +                        1                                            -                      j                                        )                                    ,                                      (                                                                  -                        1                                            -                      j                                        )                                    ,                                      (                                                                  -                        1                                            +                      j                                        )                                    ,                                                                                                                          (                                                                  -                        1                                            -                      j                                        )                                    ,                                      (                                                                  -                        1                                            +                      j                                        )                                    ,                                      (                                                                  -                        1                                            -                      j                                        )                                    ,                                      (                                                                  -                        1                                            +                      j                                        )                                    ,                                      (                                                                  -                        1                                            -                      j                                        )                                    ,                                                                                                                          (                                          1                      -                      j                                        )                                    ,                                      (                                          1                      +                      j                                        )                                                                                                          (        4        )                                          SB                      26            ⁢            …26                          =                                            (                              13                /                6                            )                                *                                                                      {                                      0                    ,                    0                    ,                                          1                      +                      j                                        ,                    0                    ,                    0                    ,                    0                    ,                                                                  -                        1                                            -                      j                                        ,                    0                    ,                    0                    ,                    0                    ,                                                                                                                                            1                    +                    j                                    ,                  0                  ,                  0                  ,                  0                  ,                                                            -                      1                                        -                    j                                    ,                  0                  ,                  0                  ,                  0                  ,                                                                                                                                                -                      1                                        -                    j                                    ,                  0                  ,                  0                  ,                  0                  ,                                      1                    +                    j                                    ,                  0                  ,                  0                  ,                  0                  ,                  0                  ,                  0                  ,                  0                  ,                  0                  ,                                                                                                                                                -                      1                                        -                    j                                    ,                  0                  ,                  0                  ,                  0                  ,                                                            -                      1                                        -                    j                                    ,                  0                  ,                  0                  ,                  0                  ,                                      1                    +                    j                                    ,                                                                                                      0                  ,                  0                  ,                  0                  ,                                      1                    +                    j                                    ,                  0                  ,                  0                  ,                  0                  ,                                      1                    +                    j                                    ,                  0                  ,                  0                  ,                  0                  ,                                                                                                                                                1                      +                      j                                        ,                    0                    ,                    0                                    }                                                                                        (        5        )                                          SB                                    -              26                        ⁢            …26                          =                                                            {                                                      +                    1                                    ,                                      +                    1                                    ,                                      -                    1                                    ,                                      -                    1                                    ,                                      +                    1                                    ,                                      +                    1                                    ,                                      -                    1                                    ,                                      +                    1                                    ,                                      -                    1                                    ,                                      +                    1                                    ,                                                                                                                          -                  1                                ,                                  +                  1                                ,                                  +                  1                                ,                                  +                  1                                ,                                  +                  1                                ,                                  +                  1                                ,                                  +                  1                                ,                                  -                  1                                ,                                  -                  1                                ,                                  +                  1                                ,                                                                                                          +                  1                                ,                                  -                  1                                ,                                  +                  1                                ,                                  -                  1                                ,                                  +                  1                                ,                                  +                  1                                ,                                  +                  1                                ,                0                ,                                  +                  1                                ,                                  -                  1                                ,                                                                                                          -                  1                                ,                                  +                  1                                ,                                  +                  1                                ,                                  -                  1                                ,                                  +                  1                                ,                                  -                  1                                ,                                  +                  1                                ,                                  -                  1                                ,                                  -                  1                                ,                                  -                  1                                ,                                                                                                          -                  1                                ,                                  -                  1                                ,                                  +                  1                                ,                                  +                  1                                ,                                  -                  1                                ,                                  -                  1                                ,                                  +                  1                                ,                                  -                  1                                ,                                  +                  1                                ,                                  -                  1                                ,                                                                                                                              +                    1                                    ,                                      +                    1                                    ,                                      +                    1                                    ,                                      +                    1                                                  }                                                                        (        6        )            
Contents of SA, SB and SC are shown as above.
The series A is the unit of the repetition of first 16 samples in the output signal to be put out from the IFFT when the SA is entered into the IFFT.
Furthermore, the series B is the unit of repetition of the first 16 samples in the output signal to be put out from the IFFT when the SB is entered into the IFFT.
Furthermore, the series C is the unit of repetition of 64 samples of output signal to transmitted from the IFFT or 16 samples going back to the past along the time axis from the last one of the 64 samples when SC is entered into the IFFT.
The reference symbol of each digital communication system is comprised of the series A and series B combined.
In this connection, in IEEE802.11a, the reference symbol is formed as shown in FIG. 4, and the reference symbol shows the waveform as shown in FIG. 5. Moreover, in the BRAN for BCH, the reference symbol is formed as shown in FIG. 6, and the reference symbol shows the waveform as shown in FIG. 7. Furthermore, in the BRAN for UL, the reference symbol is formed as shown in FIG. 8, and the reference symbol shows the waveform as shown in FIG. 9. And in the high speed wireless access system (HISWAN) for UL, the reference symbol is formed as shown in FIG. 10, and the reference symbol shows the waveform as shown in FIG. 11.
Incidentally, in order to detect the reference symbol, the sync detection using the correlator having the construction shown in FIG. 19 has been invented, not having the construction of the reference symbol. And since C region of the reference symbol is used for assuming the transmission path, same waveform is used commonly in all digital communication systems.
At this point, the amplitude of correlator output (the top stage) takes the value from 0 to 1. The plain 1 value continues and the shape having sharp peaks appears. However, these waveforms differ slightly from each other. The amplitudes of the real part and the imaginary part are shown in the middle stage and the lower stage. The real part takes the value from +1 to −1, and according to the combination of codes at the peak position of the top stage, the difference of preamble occurs as shown in FIG. 15.
However, although it is difficult to differentiate between the IEEE802.11a and the UL of BRAN, the distinction between the IEEE802.11a and the BCH of BRAN is possible, and it is considered that this causes no problem. The imaginary part takes the value of 0. If the frequency difference occurs, the imaginary part gets the value, and thus, the frequency difference trap will be conducted to make this value 0 as described in the Japanese Patent Application No. 10-330208 (Japan Patent Laid-Open No. 11-215097 bulletin).
Next, a flow of the general synchronizing operation and the OFDM demodulation operation will be shown in FIG. 16. In FIG. 16, firstly, the existence of signal will be detected by detecting the reference symbol. This is not only the detection of signal existence by detecting the electric power of the received signal, but also, by detecting the signal waveform pattern specific to the digital communication system, we can know that the signal is the communicating party of our digital communication system. Then, after the detection of reference symbol is being conducted, the data demodulation will be conducted according to the OFDM system.
In the conventional digital communication system of the OFDM system, there are two problems and these will be described in the following paragraphs.
Firstly, as the first problem supposing that the same reference symbol is used in all digital communication systems, these would not be known as different digital communication systems after these are demodulated by the OFDM system. Thus, this causes a problem that the wireless terminal has to conduct the time-wasting operation.
Furthermore, in the case where the OFDM systems of different digital communication systems are different systems, frames can be synchronized but the signal cannot be demodulated by the OFDM system. As a result, the sync signal arrived cannot be identified as to whether it is for the wireless home link or the high speed wireless access and the hang-up may occur.
Since such circumstance is not favorable for designing the digital communication system, it is necessary to use the frame sync signal different from the conventional one in the wireless home link. Similarly, regarding the packet sync signal to be used in each packet in the frame not only to the frame sync signal, it is necessary to use the sync code series different from other digital communication systems. Thus, according to the present invention, it is necessary to construct a new sync code series that can be identified from the other digital communication systems by using the same correlation circuit as before.
On the other hand, the second problem is that the lengths of reference symbols are different. In the IEEE802.11a and BRAN, HISWAN, the length of one OFDM symbol is formed of 80 samples, i.e., 4 microsecond, and the length of reference symbol is formed of an integral multiple of it, that is, the length of reference symbol is formed of 320 samples.
However, as the length of OFDM symbol in the wireless home link, 72 samples per unit is proposed. And in this case, the reference symbol is desirable to have the integral multiple of 72 samples. For example, if it is used for 4 symbols, it becomes 288 samples (14.4 microsecond). If the conventional reference symbol is used as it is, it becomes 1.6 microsecond longer, and because not only the transmission efficiency would be decreased but also the processing cannot be conducted by dividing per one symbol, the construction of timing generation mechanism of the transmitter becomes complicated.
Accordingly, it is necessary to have the series with the length of reference symbol 14.4 microsecond. And it is desirable that this series can be detected in the same correlation circuit as the conventional circuit because of ease of forming LSI. Since these are the digital communication systems using the same 5 GHz band, it is expected to design the common machine that can be used in a plurality of systems.
In that case, if the structures of the correlation circuits differ according to the digital communication system to be used, the circuit size of LSI becomes large and the unit price of the LSI will not be decreased. And also if the common correlation circuit could be used, the same LSI can be used when making the single mode device not only the common machine. And thus, it is expected that the unit price of LSI can be lowered.
Accordingly, in the digital communication system, it is desired to have the new reference symbols having completely different structures that can be detected distinguishing them from each other and having the length of the integral multiple of 3.6 microsecond.